This invention concerns methods for making clear barrier membranes for cushioning devices for shoes. In particular, the invention relates to transparent, resilient laminate membranes including a thermoplastic polyurethane elastomer.
Barrier membranes and inflatable bladders formed from such membranes have been used in footwear. It is often desirable to use thermoplastic polymeric materials to form the membranes because thermoplastic materials may be reclaimed and reformed into new articles, reducing waste during manufacturing operations and promoting recycling of scrap. Barrier membranes for inflated bladders can thus be made with a thermoplastic barrier layer. Thermoplastic polymeric barrier layer materials typically form crystalline regions or spherulites that serve to make the egress of fluid molecules through the layer more difficult.
Thermoplastic polymeric barrier materials with at a thickness adequate to provide the desired low gas transmission rate [GTR] generally do not have a low enough modulus for cushioning in shoes because the inflated bladder is subjected to high strains during use. In order to overcome this problem, the barrier materials have been blended or layered with elastic materials. Elastic materials, or elastomers, are able to substantially recover their original shape and size after removal of a deforming force, even when the part has undergone significant deformation. Elastomers may likewise be thermoplastic, and so a flexible, thermoplastic barrier film may be formed with a combination of thermoplastic elastomers and thermoplastic barrier layer materials.
In footwear, styling considerations have made low haze, transparent barrier membranes desirable. A blend material may be cloudy or hazy, however, if the materials blended together in a layer are not entirely compatible. A related problem arises in recycling scrap membrane material. When a membrane has been constructed with layers of different materials, those materials are not easily separated in the scrap. Consequently, the multi-layer scrap must be blended into one or another of the layer materials for recycling. For the desired clarity to be preserved in the blended layer, the multi-layer scrap material must be compatible with the layer material into which it is blended.
One type of thermoplastic elastomer that has been blended or layered with the barrier materials to make resilient membranes is thermoplastic polyurethane. Membranes including a first layer of a thermoplastic polyurethane, and a second layer including a barrier material, such as a copolymer of ethylene and vinyl alcohol, are described, for example, in U.S. Pat. No. 6,082,025, issued Jul. 4, 2000; U.S. Pat. No. 6,013,340, issued Jan. 11, 2000; U.S. Pat. No. 5,952,065, issued Sep. 14, 1999; and U.S. Pat. No. 5, 713,141, issued Feb. 3, 1998, each of which is incorporated herein by reference. Although membranes with separate layers of the thermoplastic polyurethane material and the polymeric barrier material have had acceptable clarity, recycling scrap multi-layer membrane material by blending the scrap material into one of the layers has been problematic. In particular, the blended layer and membrane have noticeably lower reduced clarity because of incompatibility of the blended materials.
Thus, it would be desirable to have a transparent, multi-layer membrane in which the layer compositions are compatible so that incorporation of the multi-layer scrap into one layer of the membrane does not result in undesirable haziness.
The present invention provides an article of footwear in which the sole includes a visible, fluid-containing bladder. To be visible, at least a part of the bladder wall forms at least a part of an exterior portion of the sole. The bladder wall comprises a laminate membrane having low haze. The laminate membrane includes at least a first layer containing a polyurethane and a copolymer of ethylene and vinyl alcohol, and a second layer containing a copolymer of ethylene and vinyl alcohol. The polyurethane includes at least about 50 mole percent, based on the total moles of hydroxyl-functional reactants used to produce the polyurethane, of a polyester diol having a weight average molecular weight of at least about 500 and having a linear alkylene group having from two to about six carbon atoms between substantially all of the ester groups.
The invention further provides a method of manufacturing footwear, in which a laminate membrane is prepared with at least a first layer containing a polyurethane and a copolymer of ethylene and vinyl alcohol, and a second layer containing the copolymer of ethylene and vinyl alcohol. Again, the polyurethane includes at least about 50 mole percent, based on the total moles of hydroxyl-functional reactants used to produce the polyurethane, of a polyester diol having a weight average molecular weight of at least about 500 and having a linear alkylene group having from two to about six carbon atoms between substantially all of the ester groups. The blend of polyurethane and copolymer of ethylene and vinyl alcohol in the first layer is made by including recycled material of the polyurethane and copolymer of ethylene and vinyl alcohol in the layer, especially along with virgin material that includes at least the polyurethane. The recycled material has a first layer including the copolymer of ethylene and vinyl alcohol and a second layer including thermoplastic polyurethane material. Because of the particular polyurethane used, the blended material is very low in haze. The low haze is desirable for the aesthetic design of the footwear. The low haze membrane can be colorless or colored with dye or transparent pigment to provide a low haze colored membrane. The bladder may be filled with a colorless or colored fluid.
The membrane preferably includes the layer of blended material as an outer membrane layer in a multi-laminar structure. The transparent membrane of the article also preferably includes a barrier layer to prevent the transfer of fluid from one side of the membrane to the other, preferably with a thermoplastic elastomer layer between the layer of the blended material and the barrier layer. Such durable, elastomeric barrier membranes may be used to prepare inflated bladders. By xe2x80x9cdurablexe2x80x9d it is meant that the membrane has excellent resistance to fatigue failure, which means that the membrane can undergo repeated flexing and/or deformation and recover without cracking and without delamination along the layer interfaces or cracking through the thickness of the membrane, preferably over a broad range of temperatures. For purposes of this invention, the term xe2x80x9cmembranexe2x80x9d is used to denote a free-standing film separating a fluid, preferably a gas at higher than atmospheric pressure, from the atmosphere outside of the article of footwear. Films that are wholly laminated or painted onto another article for purposes other than separating fluids, e.g., coatings, are excluded from the present definition of a membrane.
The layer of blended material of the invention has a low haze, by which is meant a haze of no more than about 12%, preferably no more than about 5%. Haze may be measured according to ASTM D-1003.
The transparent membrane is part of a bladder containing a fluid. The bladder may be inflated with a gas such as nitrogen, air, or a supergas. The term xe2x80x9csupergasxe2x80x9d refers to a large molecule gas that has a low solubility coefficient, such as SF6, CF4, C2F6, C3F8, and so on which are described in Rudy, U.S. Pat. Nos. 4,183,156 and 4,287,250, and Rudy et al., U.S. Pat. No. 4,340,626, incorporated herein by reference. A portion of the transparent membrane of the enclosure or bladder forms, or is visible through, at least part of an exterior wall of the article of footwear into which the bladder is incorporated.
The barrier membrane preferably has a gas transmission rate that is sufficiently low to allow the bladder to remain xe2x80x9cpermanentlyxe2x80x9d sealed and inflated, that is, to retain a useful internal pressure for the useful life of the article into which it is incorporated. An accepted method for measuring the relative permeance, permeability, and diffusion of different film materials is ASTM D-1434-82-V. The gas transmission rate of a membrane is expressed at the quantity of gas per area per time that diffuses through the membrane. The gas transmission rate may be expressed in units of (cc)(mil)/(m2)(24 hours), at standard temperature and pressure. The gas transmission rate of the barrier membrane provided by the invention is preferably less than about 1 (cc)(20 mils)/(m2)(24 hours).